1. Field of the Invention
The present invention relates generally to control systems for a compressor, and more particularly, to an anti-surge valve configured to anticipate surge conditions for purposes of reducing valve dead time on a valve seat when action is required by a compressor controller.
2. Description of the Related Art
Compressors are frequently employed in many industrial applications. One of the primary concerns associated with the operation of a compressor is compressor surge, which typically occurs in a centrifugal or axial compressor when the inlet flow is reduced to an extent such that the compressor, at a given speed, can no longer pump against the existing pressure head.
The occurrence of compressor surge may induce a reversal of gas flow through the compressor, which may be accompanied with a drop in pressure head. Eventually, normal compression resumes, although the cycle typically repeats. The occurrence of cyclical surge events may cause pulsation and shock to the entire compressor and pipe arrangement. If left uncontrolled, damage to the compressor could result.
In view of the undesirable conditions associated with compressor surge, many compressor systems include a bypass valve (e.g., an anti-surge valve), which may reroute gas flow around the compressor or exhaust gas to the atmosphere when compressor surge is imminent so as to maintain minimum flow through the compressor. During normal compressor operation, the bypass valve may remain closed. However, as the surge conditions arise, the bypass valve may selectively open with the aim of ultimately avoiding compressor surge.
The movement of most bypass valves is governed by an actuator, which provides the motive force to open and close the valve element. The actuator may also provide an additional force when the valve is in a closed position to keep the valve sealed tight so as to mitigate leakage between the upstream and downstream flows relative to the valve. The actuator may employ pneumatic, hydraulic, electrical, or mechanical energy for moving the bypass valve between the closed and open positions.
A conventional pneumatic actuator is comprised of a piston sealed within a cylinder, the piston including a connecting rod that is mechanically coupled to the valve element. Compressed gas is forced into and out of the cylinder to move the connecting rod, which is mechanically coupled to the stem of the control valve. In a single-acting actuator, the compressed gas is taken in and exhausted from one end of the cylinder and is opposed by a range spring, while in a double-acting actuator, air is taken in one end of the cylinder while simultaneously exhausting it out of the opposing end.
Precise and accurate control of the valve actuator, and hence the valve element, can be achieved with a positioner device coupled thereto. Pneumatic valve positioners, which can cooperate with the aforementioned pneumatic actuators, are well known in the art. The proportional movement of the actuator may be accomplished by the movement of compressed gas into and out of the actuator piston. More particularly, conventional valve positioners incorporate a spool (or other devices) that either rotates or slides axially in a housing to port the flow of compressed gas to the actuator or to one or more exhaust ports.
An electrical control circuit may provide a variable current signal to the positioner device that proportionally corresponds to particular states of the actuator and hence a particular position of the control valve. The electrical control circuit and the electrical current signals generated thereby may be part of a broader process managed by a distributed control system (DCS). Generally, the electrical current varies between 4 milliamperes (mA) and 20 mA according to industry-wide standards; at 4 mA, the valve positioner may fully open the valve element, while at 20 mA, the valve positioner may fully close the valve element (or the opposite, according to the logic control of the plant). The positioner compares the received electrical signal to the current position of the actuator, and if there is a difference, the actuator is moved accordingly until the correct position is reached.
Typically, between the positioner and the actuator, there is a safety shut down/trip system, which is generally piloted by a solenoid valve (SOV). The safety shut down/trip system typically operates to drive the valve to the fully open position, as fast as possible, when the SOV is de-energized, regardless of the positioner signal/action. It is industry practice that the SOV drives 3-way pneumatically operated valves to perform the fast stroke required by the trip signal. Generally, the stroking time required in opening the SOV is less than or equal to the stroking time required under controlled opening.
A common deficiency associated with conventional antisurge control systems is that there is a delay (e.g., dead time) associated with moving the valve from its closed position to its incipient open position. With the term “stroking time,” reference is made instead to the overall time the valve takes to reach its fully open position, starting from a closed position, when the opening signal is given to the valve. Along these lines, when the conditions which trigger movement from the closed position to the open position are detected, pressure within the actuator must be discharged before valve movement is to occur. This delay is typically due to the additional force given by the actuator, necessary to guarantee tightness between the plug and the seat of the valve. In this respect, the venting of such pressure from the valve actuator prevents immediate movement of the valve.
With reference to FIG. 1, in the current state of the art, the controller of the compressor typically provides a proportional signal to open the valve when an action of flow recycling is needed. Such a signal is typically the result of the combination of a Proportional+Integral signal 2 (generally smooth) and a recycle trip action 4 that is normally a step response. In reality, due to the delay of the response of the antisurge valve to a smooth and proportional signal (the discharge of the pressure of the actuator is not immediate), there is no real action of the valve until a recycle trip action is given to the valve. In that case, the control of the operating point may not be accurate. Avoidance of the action trip 4 may result in better performance of control and avoidance of loss of production.
It is current practice to take some safety margin to avoid trip occurrence. This is often done by establishing an ideal Surge Control Line (SCL) which separates itself from the Surge Limit Line (SLL) which is the line at which the surge is expected to occur. The higher the distance between the SCL and SLL, the higher the safety of compressor operation, but at a price of greater inefficiencies, since a part of the compressor map showing good efficiency cannot be used.
Accordingly, there is a need in the art for an improved anti-surge valve that is capable of reducing dead time of the piston actuator when surge conditions occur.